This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Most pipelines used for the transportation of oil, gas, water, or mixtures of these fluids, are constructed from carbon steel. Carbon steel is a desirable material due to its availability, low cost relative to other materials, strength, toughness and ability to be welded. However, carbon steels can be corroded by many of the fluids contacting them. Almost all carbon steel pipelines have some level of corrosion of their internal surface and large costs are expended in the monitoring of corrosion, injecting chemicals into the pipeline to inhibit corrosion, and inspection of the pipeline.
Even with these mitigating activities, significant corrosion can occur, causing reduction of the pipe wall thickness, typically in uneven channels or pits. The corrosion can extend along long segments of a pipeline or may be only in localized areas. Furthermore, the corrosion may grow through the pipeline wall resulting in small leaks. These leaks are typically repaired by applying an external clamp around the pipeline. At times the corrosion can be so extensive that external clamps are ineffective and segments of the pipeline are replaced at high cost, often causing long term deferred production of hydrocarbons.
Pipeline liners have been used to provide a barrier against the deleterious effects of internal corrosion on pipelines. The plastic materials of the pipeline liners are placed in direct contact with the transported fluids instead of the steel pipeline. The liners exhibit superior corrosion resistance, yet provide a cost-effective alternative to pipeline replacement or the use of corrosion-resistant alloys. Additionally, remediation of a deteriorated pipeline with a pipeline liner can allow restoration of the full pressure rating of the pipe.
The market for liners is mature to the point that several competing technologies are available. Several types of liners are intended for use in the water-transport and sanitation markets, providing short-length rehabilitation within the pipeline. The vast networks of pipelines in the oil and gas industry have facilitated the development of several long distance pipeline liner options.
Types of long distance pipeline liners include thermoplastic liners and composite liners. Both thermoplastic and composite liners provide corrosion resistance when installed, but the variations in mechanical properties make each of them attractive for particular applications.
Thermoplastic liners, which are the more simple form of pipeline liners, are composed entirely of polymeric, or plastic, material. The most commonly used polymer in pipeline liner applications is High-Density Polyethylene (HDPE), due to its low cost, availability, and range of service conditions. Alternative plastics may also be selected for their enhanced strength or high-temperature service capabilities. These thermoplastic materials have excellent formability and advantageous material properties. Thermoplastic liners are generally not strong enough to withstand long pull lengths or independently withstand the full range of operating pressures prevalent in the hydrocarbon production industry.
Thermoplastic feedstock can easily be extruded into continuous tubular forms. Precise dimensional control allows the liner to conform to the host pipe. The pipeline liner can be reeled for delivery if it has a small diameter, or the liner segments can be fusion welded on-site. Insertion of the liner, or slip-lining, often necessitates that the plastic liner have a temporary size reduction in order to easily traverse within the host pipeline.
Thermoplastic properties allow several options for this size reduction, including roller reduction and folding of the tube into a smaller diameter. In service, the host pipe is still relied upon for pressure containment, but the strength of thermoplastics does allow bridging of small gaps, pits, or pinholes. However, the relatively low range of mechanical strength properties of thermoplastic liners does impose other limitations. The low longitudinal strength limits the pulling length, as the liner will tear under its own weight and the frictional drag that arises during slip-lining. It also limits the available host pipe geometries; typical minimum bend radii are on the order of 50 pipe diameters.
Composite liners are another major category of pipeline liners. Composite liners have been developed to expand the range of conditions in which liners may be applied. The cost of composite liners may prohibit their use in remediation projects if the full extent of their properties is not necessary, such as a short pipe that is still capable of pressure containment.
Currently available composite liners are manufactured in a multi-step process in which successive layers are wrapped around a plastic core pipe. In this way, the corrosion resistance of thermoplastics can be combined with the mechanical properties afforded by reinforcing materials such as glass fiber, metallic cables or wires, carbon fiber, ultra-high molecular weight polyethylene (UHMWPE), or nylon. The complexity of these systems necessitates more tooling and results in a greater cost per unit length over plastic liners, but the superior mechanical properties grant the tubing sufficient hoop strength for pressure-containment. In many cases, the host pipe only serves as a conduit for running the composite liner, which then acts as a self-sufficient pipeline. Many composite liners available in the market today were initially designed as stand-alone flexible pipe. The complex fabrication of these composites typically requires that they be manufactured in a facility and then delivered to the installation site on a spool. The size of spools which can be delivered onshore can limit composite liners to small (<6″) diameters.
Like thermoplastic liners, composite pipe liners are installed via slip-lining. The high strength properties allow much longer insertions. The high strength also permits composite liners to negotiate sharper bends in the host pipe. Some known composite liners permit a minimum bend radius as low as nine (9) pipe diameters.
One specific known composite pipeline liners employs an inner HDPE pipe wrapped in various layers of reinforcement. This liner was originally conceived to overcome some of the challenges inherent in the lining process by fabricating the composite in the field. The portable factory removes the length limitations that reeling imposes on length (up to 10 miles), and allows for significantly larger diameter pipelines to be lined. In general, existing liner technologies have not been shown to overcome the issue of severe bends (three to five diameters) in the host pipeline.
As noted above, the insertion of such pipeline liners typically involves slip-lining the liner within an existing host pipe. Unfortunately, this process may result in local damage on the liner. Once inserted into the host pipe, it is often extremely difficult, if not impossible, to locate the damage on the liner. After the pipeline liner is installed, the commissioning process often involves a hydro-test in which the liner and host pipe are filled with water at high pressure and are tested for leaks. In the event that leaks are identified, the leaking section or portion of the liner is repaired or replaced. However, identifying the existence of leaks and/or locating such leaks are challenging processes.
Further, because the pipeline liners are primarily constructed of plastic (or derivative) materials, it is not possible to use conventional in-line inspection (ILI) tools. As a result, it also becomes difficult to understand the structural integrity of the liner and pipeline during operations. Because pipeline liners are typically installed in old pipelines in need of rehabilitation, integrity monitoring of such pipes is critical.
Thus, there is a need for improvement in this field.